The use of organic solvents or other volatile hydrocarbons in processing chemicals for industrial and consumer use is generally known. Solvents known to be useful include organic compounds like hexane, heptane, toluene, benzene, and other hydrocarbons. Within these processes, organic solvents may come into contact with water. Water thus contaminated with an organic solvent must be treated prior to disposal or reuse by removing the organic solvent to meet regulations targeted to prevent detrimental environmental impacts.
It is known that many of the organic solvents or volatile hydrocarbons are only slightly miscible or partially miscible in water. Thus, the most common way to separate at least a portion of the organic solvent from the water is by gravity or phase separation in which the relatively low solubility of the solvent in the water causes the solvent to separate from the water and form a solvent layer and a water layer with a phase boundary therebetween. However, for purposes of waste water treatment, the separation by gravity is insufficient to purify the water to a point where it may be safely discharged to the environment. Small concentrations of organic solvents or volatile hydrocarbons are soluble in the water. For example, the concentration of hexane in water ranges from 0.02 wt. % at 70.degree. F. to 0.08 wt. % at 110.degree. F. To meet existing and proposed environmental regulations, the concentration of organic solvents must be reduced to near zero before discharging a waste water stream.
Another known property of organic solvents or volatile hydrocarbons in water systems is that many organic solvents form an azeotrope with water at certain concentrations. Thus, the azeotrope prevents a complete separation of the organic solvent from water by heating. Using a hexane/water system as an example, while water boils at 212.degree. F. at atmospheric pressure and hexane boils at 156.degree. F. at atmospheric pressure, a hexane water azeotrope which boils at approximately 142.degree. F. is formed. The azeotrope concentration is approximately 94% hexane and 6% water. Thus, to reduce the concentration of the solvent to near zero by heating a liquid stream and removing a vapor containing most of the solvent, a concentration of water at least equal to the azeotropic concentration must also be removed with the solvent.
An example of a process which utilizes an organic solvent which becomes mixed with water is the process utilized to extract oil from soybeans. In this process, it is known to use hexane to extract soybean oil from the seed after it is ground. The spent solvent contains water and is phase separated to remove most of the hexane from the water. However, a small quantity of hexane is soluble in the water. This waste water must be treated prior to discharge into a sewer, lake or river to strip the water of all significant traces of hexane.
One prior art device utilized to treat the solvent containing waste water stream includes an enclosed vessel into which the waste water is pumped proximate the bottom of the enclosed vessel. A wall extends upward from the bottom of the vessel over a portion of the height to separate the lower portion of the internal volume into two areas. As the waste water enters the first side of the internal volume of the vessel, sparge steam or direct steam is added to the water to heat it rapidly. This heat causes some of the hexane to vaporize. The remaining water is allowed to exit the vessel through the other side of the internal volume.
Applicants have found that such system requires operating temperatures as high as 205.degree. F. to remove substantially all of the solvent in a hexane/water system. This requires an excessive amount of energy and is not economically efficient. Finally, the design of the apparatus gives uneven flow and low surface contact area which contributes to the device's inefficiency. Further, the device is unable to compensate for an upset in which a larger quantity of solvent enters the treatment process and could be separated by phase separation.
Another known apparatus also utilizes an enclosed vessel with counter-current flow of liquid and vapor in a vertical direction to separate the solvent. Liquid in the bottom of the vertical vessel is heated to vaporize a portion of the solvent and water. This design also includes trays which extend part of the way across the width of the vertical vessel to cause mixing of the streams being separated. This system also requires high energy input to reduce the concentration of solvent to acceptable levels due to the inefficiency created by low surface area contact. Further, this system does not compensate for upsets in which the concentration of the solvent may be increased to the point where phase separation would assist in purifying the water stream.
Accordingly, the need exists for a method and apparatus for efficiently reducing the solvent concentration or volatile hydrocarbon concentration in a waste water stream to near zero. An apparatus and method to remove such solvents should provide high surface area to increase the efficiency of separation and should incorporate features which allow phase separation of the solvent from the water by gravity during an upset in which the solvent concentration is greater than the solubility of the solvent within the water. Further, the apparatus should minimize energy consumption by incorporating features which allow complete separation of the azeotrope at reduced temperatures due to increased efficiency of separation. Finally, the apparatus should be designed as a simple, relatively compact unit incorporating each of the above features so that a user may readily install such treatment device for use on existing waste water streams and associated equipment with minimal capital investment.
The present invention addresses these needs as well as other problems associated with existing methods and apparatus for treating contaminated water streams containing a partially miscible solvent. The present invention also offers further advantages over the prior art and solves problems associated therewith.